The images taken by Hubble Space Telescope and from the best telescopes on Earth show how Pluto and Charon are mere specks in the sky - and we see very little detail of their surfaces. To illustrate how "fuzzy" our view is of Pluto, below is a comparison with an image of the Moon degraded to the same resolution.
Pluto at best HST resolution
Earth's Moon at the same resolution
But astronomers were lucky that a quirk of nature - mutual eclipses of Pluto and Charon -provided an opportunity to make better maps of Pluto. They were clever enough to seize the rare opportunity.
Pluto is tipped on its side with its spin axis close to the plane of its orbit. Like most satellites, Charon orbits in Pluto's equator. From 1985 through 1990, Pluto's equator and Charon's orbit plane were aligned with the line-of-sight from Earth, with Charon alternating between passing in front of and behind Pluto every 3.2 days. This means that from 1985 through 1990 Pluto and Charon eclipsed each other every Pluto day.
These eclipses turned out to be very important, since observations of the eclipses (called "mutual events") led to the first accurate determination of Pluto's and Charon's sizes as well as a map of the reflectance of Pluto's surface, and separate compositions of Pluto (methane-rich) and Charon (water ice-rich). While Charon moved in front of and then behind Pluto every 6.4 days, the location of Charon - and Charon's shadow - varied slowly over the 1985-1990 eclipse season.
Astronomers stared at Pluto as Charon moved in front and carefully measured the dimming of sunlight reflected by the combined objects due to Charon blocking some of Pluto's surface. At the beginning and end of the eclipse season Charon only blocked out a very little of the light and there was little change in the "light curve." Around the peak of the eclipses, in 1987, the reflected sunlight dimmed by more than 30%.
Several maps of Pluto's albedo (surface reflectivity) have been made based on the mutual event light curves. By keeping careful track of the decrease in brightness observed when part of Pluto is covered by Charon, we can solve for the brightness of the covered part. Eventually we can piece together a mosaic of Pluto's Charon-facing hemisphere.
This map shows a bright south polar region, which we now know to be frost made mostly of nitrogen, with some methane. The dark features have not been identified, although they may be colored by methane's photochemical byproducts.
Recent images of Pluto from the Hubble Space Telescope show an icy, mottled, dark molasses-colored world undergoing seasonal surface color and brightness changes. Pluto has become significantly redder, while its illuminated northern hemisphere is getting brighter – most likely consequences of surface ice sublimating from on the sunlit pole and then recondensing on the other pole, as Pluto heads into the next phase of its 248-year-long seasonal cycle.
When Hubble pictures taken in 1994 are compared to those of 2002 and 2003, astronomers see evidence that the northern polar region has gotten brighter, while the southern hemisphere darkened. These changes hint at very complex processes affecting the visible surface.
New Horizons science team member Marc Buie, who led this mapping effort, says the Hubble observations are the key to tying together diverse constraints on Pluto and showing how it all makes sense by providing a context based on weather and seasonal changes, which opens other new lines of investigation.
The Hubble images surface variations a few hundred miles across that are too coarse for understanding surface geology. But in terms of surface color and brightness, Hubble reveals a complex-looking world with white, dark-orange and charcoal-black terrain. The overall color is believed to be a result of ultraviolet radiation from the distant sun breaking up methane present on Pluto's surface, leaving behind a dark and red-carbon-rich residue.
The Hubble images are a few pixels wide. Through a technique called dithering, multiple, slightly offset pictures are combined through computer-image processing to synthesize a higher-resolution view than can be seen in a single exposure.
"This has taken four years and 20 computers operating continuously and simultaneously to accomplish," Buie said in 2010, when the maps were released. Buie developed the special algorithms to sharpen the Hubble data. He planned to use Hubble's Wide Field Camera 3 to make additional observations prior to the arrival of New Horizons in July 2015.